In the pulp processing industry, wood chips are introduced into a digester where these are treated under high pressure and temperature by a liquor that is introduced into the digester to break down the lignin and sugar content of the wood fibers, leaving only the cellulose. During the digesting process, the liquor is drawn out of the digester at various locations and recirculated to other locations in the digester. So that the wood chips remain in the digester, it is necessary that the liquor that is being removed passes through a strainer which has slot like openings that prevent the passing of wood chips but permit the passage of the liquor therethrough.
If the screens in the digester become clogged, then it is commonly necessary to shut down the digester and remove the scaling from the strainers. Such a shutdown can be extremely costly, and make it economically unfeasible to operate the pulp mill in that manner. Quite commonly this problem is alleviated by formulating the composition of the liquor so that certain components of the liquor will prevent the formation of the scaling on the strainers.
This formulation will depend to some extent on the species of the wood from which the chips are made. For example, if the wood has a high percentage of pitch, the scaling would quite possibly be more of a problem. An example of this is in a digester which uses alcohol (either ethyl or methyl alcohol) as one of the major components of the liquor. If soft wood is being treated in the digester, there is more of a tendency for scaling to form. However, the addition of sodium hydroxide to the liquor composition substantially alleviates the scaling problem.
The further treatment of the black liquor resulting from the digester process is also an important part of the operation of a pulp mill. In order to operate a pulp mill economically, it is generally necessary to process or utilize the black liquor in some manner to extract value therefrom. This can be done in various ways. Generally, the black liquor goes to an evaporator where a substantial portion of the water content is removed. Then the residue from the black liquor can be burned to generate heat energy which is utilized in other parts of the pulp mill and to recover the non-organic chemicals in the liquor for recirculation in the pulping process. Alternatively, the residue remaining after the evaporation of the black liquor can be utilized in other applications, such as producing adhesive for particle board or animal food pelletizing.